1. Field of the Invention
The present invention relates to a vehicle drum brake apparatus, and particularly to improvement of a shoe hold-down mechanism for holding a brake shoe on a backing plate so that the brake shoe does not lift from the backing plate.
2. Background Art
According to a conventional drum brake apparatus, in a shoe hold-down mechanism for holding a brake shoe on a backing plate so that the brake shoe does not lift from the backing plate, there has been widely used a hold-down pin which is fixed to the backing plate through a web of the brake shoe, a spring receiver locked onto a head side of the hold-down pin protruding from the web, and a helical compression spring which lies between the spring receiver and the web in a compressed state thereby to urge the web to the backing plate side.
However, such the shoe hold-down mechanism causes increase in number of parts and increase in number of assembly steps, so that cost increase of the drum brake apparatus is caused.
Therefore, as shown in FIGS. 10 and 11, a drum brake apparatus having an improved shoe hold-down mechanism has been developed.
A drum brake apparatus 101 shown in FIGS. 10 and 11 has been disclosed in JP-U-02-101138, which constitutes a shoe hold-down mechanism by a shoe guide portion 105 formed integrally with a backing plate 103 by cutting and press molding, and engagement projections 107b, 108b formed integrally with rims 107a, 108a of a pair of brake shoes 107, 108 arranged on the backing plate 103 oppositely to each other.
The shoe guide portion 105 is formed integrally with the backing plate 103 by the cutting and press molding so that its rear surface becomes a flat guide surface 105a which is parallel to the backing plate 103.
Further, the engagement projections 107b, 108b are formed integrally with each rim 107a, 108a so that their upper surfaces become parallel to the backing plate 103. The engagement projections 107b, 108b engage with the guide surface 105a slidably, whereby each brake shoe 107, 108 is prevented from lifting from the backing plate 103.
Such the shoe hold-down mechanism, compared with the conventional shoe hold-down mechanism using the hold-down pin fixed onto the backing plate through the web of the brake shoe, becomes small in the number of parts and the number of assembly steps. Therefore, cost reduction of the drum brake apparatus can be realized.
However, the shoe guide portion 105 in the drum brake apparatus 101 shown in FIGS. 10 and 11, since both end edges 105b, 105c of the shoe guide portion 105 in a circumferential direction of the drum brake apparatus are cut off from the backing plate 103, it is difficult to secure rigidity which is enough so that the shoe guide portion 105 is not substantially deformed by bending moment M. Therefore, there is fear that parallelism between the guide surface 105a and the backing plate 103 lowers due to the deformation of the shoe guide portion 105 thereby to cause lowering of uplift preventing performance of the brake shoes 107, 108, or that the smooth slide of the brake shoes 107, 108 on the backing plate 103 is impaired.
Further, since the shoe guide portion 105 is formed by the cutting and press molding, an aperture 109 is formed in the backing plate 103 on the lower side of the shoe guide portion 105.
If this aperture 109 is left open, rain water readily enter from the outside into the inside, so that there is fear that braking performance is affected by entry of the rain water, or that internal parts become dusty.
Therefore, it is necessary to join a sealing plate material which boards up the aperture 109 onto the rear surface of the backing plate 103. However, depending on the joint structure, there is fear that various problems which impair productivity are produced.
For example, in case that welding is adopted as joint means, such a limit on step that a surface treatment step of the backing plate must be performed after welding of the sealing plate material is produced, so that process control management becomes difficult.
Further, in case that screwing or riveting is adopted as the joint means, a through-hole for insertion of a screw or a rivet must be provided in advance in the backing plate 103 and the sealing plate material. Therefore, there is fear that the increase in machining cost by these machining causes increase in cost of products, or that productivity lowers due to increase in number of machining steps.